Preparation For Crust:Cut shortening into flour and salt until crumbles form. Make well in center and add 1 tablespoon of water at a time and toss with a fork until flour becomes wet. Add each additional tablespoon of water. Flour should be completeley moistened and forming a ball. Divide ball in half and roll each half to 2-inches larger than an inverted pie plate. Gently place one shell into pie plate. Save other shell for top.For Pie Filling:Heat oven to 425*. Mix flour and sugar. Combine rhubarb and peaches and place half of the mixture in the pie shell. Sprinkle half of the flour-sugar mixture on top. Repeat with remaining rhubarb-peach mixture and then flour-sugar mix. Dot with margarine/butter. Cover with top pie crust. Cut slits in top of crust; seal and flute crust. Cover edge with 3-inches of foil to prevent browning. Remove for last 15 minutes of cooking. Bake at 425* for 40-50 minutes or until crust browns and juice begins to bubble through crust. Serve with vanilla ice cream or whipped cream.